Various types of image forming apparatuses for forming an image on a recording medium on the basis of an electrostatic latent image (charge pattern) formed on a photosensitive body, have been conventionally proposed. Among the proposed apparatuses, for example, an image forming apparatus disclosed in Japanese Laid-open Patent Application (Tokukaisho) No. 49-24139/1974 adopts a so-called electrostatic transfer system of developing an electrostatic latent image after transferring the electrostatic latent image formed on a photosensitive body to a recording medium by transfer means. The following description will explain this image forming apparatus.
As illustrated in FIG. 13, the image forming apparatus of this document includes a photosensitive body 101 for carrying an electrostatic latent image thereon. The photosensitive body 101 is formed by three layers, namely, a high insulating layer 101a, photoconductive layer 101b, and electrode layer 101c, arranged in this order from the outer side.
Disposed around the photosensitive body 101 is a primary corona discharger 102 for charging the surface of the photosensitive body 101 by applying a voltage of a predetermined polarity to the surface of the photosensitive body 101 by corona discharge, a secondary corona discharger 103 for charging the surface of the photosensitive body 101 by applying a voltage of the opposite polarity to that of the voltage applied by the primary corona discharger 102, to the surface of the photosensitive body 101 by corona discharge, and an entire-surface exposing light source 104.
With the use of the first corona discharger 102 and secondary corona discharger 103 which are not in contact with the photosensitive body 101 as means for charging the photosensitive body 101, it is possible to prevent movement of charges on the surface of the photosensitive body 101 and lowering of the strength of an electrostatic latent image due to the movement of charges, which occur in a structure where the surface of the photosensitive body 101 is charged by bringing the electrode into contact with the photosensitive body 101.
A transfer section 105 is provided on the downstream side of the entire-surface exposing light source 104, in a rotating direction (clockwise direction in FIG. 13) of the photosensitive body 101. The transfer section 105 includes a transfer belt 106 pressed against the photosensitive body 101 via a transfer sheet 110, and two transfer rollers 107a, 107b for stretching the transfer belt 106 therebetween. Provided on the sheet feeding side of the transfer section 105 are feed rollers 108a, 108b for transporting the transfer sheet 110 to the transfer section 105 while sandwiching the transfer sheet 110 therebetween. On the other hand, provided on the sheet output side of the transfer section 105 are feed rollers 109a, 109b for outputting the transfer sheet 110 having an electrostatic latent image transferred thereto from the transfer section 105 and transporting the output transfer sheet 110 to a developing section (not shown) while sandwiching the transfer sheet 110 therebetween.
In this structure, when the surface of the photosensitive body 101 is uniformly charged in a predetermined polarity by the primary corona discharger 102, a light image is projected onto the photosensitive body 101 while applying a voltage of the opposite polarity by the secondary corona discharger 103, thereby forming on the highly insulating layer 101a of the photosensitive body 101 an electrostatic latent image corresponding to the light image. Subsequently, the surface of the highly insulating layer 101a is illuminated with a light beam from the entire-surface exposing light source 104 so as to release permanent internal polarization in the photoconductive layer 101b. As a result, the variation of charge in the photosensitive body 101 is stabilized immediately, and the electrostatic latent image formed according to the light image is stabilized.
Thereafter, the electrostatic latent image is transported to a transfer region between the photosensitive body 101 and the transfer belt 106 by a rotation of the photosensitive body 101, and transferred to the transfer sheet 110 transported to the transfer region by the transport rollers 108a, 108b, and the transfer belt 106. After separating the transfer sheet 110 from the photosensitive body 101, the transfer sheet 110 is output from the transfer section 105 by the transport rollers 109a, 109b, and transported to a developing section.
By the way, in such an image forming apparatus adopting the electrostatic transfer system, it is likely that distorted electrostatic latent image is transferred to the transfer sheet 110 due to non-uniformity of the contract pressure between the transfer belt 106 and photosensitive body 101, etc. As a result, variations in the strength of the transferred image on the transfer sheet 110 occur. Moreover, in the above-mentioned structure, since development is performed after the transfer of the electrostatic latent image to the transfer sheet 110, the transfer sheet 110 is likely to get dirty.
In resent years, therefore, development of image forming apparatuses adopting a development transfer system, in which an electrostatic latent image formed on a photosensitive body is developed into a visible image in advance on the photosensitive body with a developer such as toner and then the visible image is transferred to a recording medium, has been actively carried out.
However, in an image forming apparatus of typical structure employing the development transfer system, as illustrated in FIG. 14, for example, when a high electric potential is applied to a transfer roller 201 as transfer means so as to perform image formation at a high speed, toner 204 on a photosensitive body 203 flies toward a sheet 202, i.e., so-called scattering of toner 204 occurs, before the sheet 202 is transported to a transfer position between the transfer roller 201 and the photosensitive body 203 and comes into contact with the photosensitive body 203. As a result, the outline of the transferred toner image is blurred, and thus the image quality deteriorates. The following description will explain the theory of occurrence of the scattering of toner.
Here, it is assumed that a straight line OQ' connecting a center O of the rotation axis of the transport roller 201 and a point Q' at which scattering of the toner 204 occurs, the point Q' being located on the upstream side of a nip region of the surface of the photosensitive body 203 along a rotating direction (clockwise direction in FIG. 14) of the photosensitive body 203, is divided into two straight lines OP' and P'Q' by a point P' on the surface of the transfer roller 201, and regions including the straight lines OP' and P'Q' are denoted as region P and region Q, respectively.
Moreover, suppose that the lengths of the straight lines OP' and P'Q' are d.sub.p and d.sub.Q, the electric potentials applied to the regions P and Q are V.sub.P and V.sub.Q. and electric fields formed in the regions P and Q are considered to be formed along the straight lines OP' and P'Q' as shown in FIG. 14 and denoted by E.sub.p and E.sub.Q, respectively, for simplification purposes, the electric fields E.sub.P and E.sub.Q are given by the lengths d.sub.p and d.sub.Q and the electric potentials V.sub.P and V.sub.Q as indicated below. EQU E.sub.P =V.sub.P /d.sub.p EQU E.sub.Q =V.sub.Q /d.sub.Q
By the way, supposing that the regions P and Q are arranged in series, the electric potentials applied to the regions P and Q are substantially proportional to the resistances in the regions P and Q, respectively. Here, the region Q is the atmosphere, and exhibits substantially a high resistance like an insulator, while the region P is formed by an elastic body, such as rubber, whose volume resistivity is, for example, around 10.sup.7 to 10.sup.8 .OMEGA..multidot.cm. Specifically, the resistance is greater in the region Q than in the region P. Therefore, when a transfer electric potential is applied to the transfer roller 201 from a transfer power supply (not shown), most of the transfer electric potential is applied to the region Q. Namely, V.sub.P &lt;&lt;V.sub.Q.
As described above, when a high electric potential is applied to the transfer roller 201, the difference between V.sub.P and V.sub.Q is greater than the difference between d.sub.P and d.sub.Q on the upstream side of the nip region, and therefore E.sub.P &lt;&lt;E.sub.Q. As a result, the electric field E.sub.Q in the region Q becomes greater than a necessary value, and the toner 204 on the photosensitive body 203 flies toward the sheet 202 before the toner 204 reaches the nip region.
Besides, in the above-mentioned image forming apparatus, as illustrated in FIG. 15, when a sheet 202 of a small width like, for example, a post card, is used, the transfer roller 201 comes into contact with the photosensitive body 203 in a region where the sheet 202 is not present. In FIG. 15, a region R is a region of the photosensitive body 203, corresponding to the width of the sheet 202, while a region S substantially corresponds to the contact region to the transfer roller 201.
Hence, when a high electric potential is applied to the transfer roller 201 as mentioned above, most of the transfer electric potential applied to the transfer roller 201 is concentrated in the region S, a transfer current flows and is concentrated in the region S, and thus a sufficient transfer current cannot be obtained on the sheet 202. Consequently, a sufficient transfer cannot be achieved. Such a problem is more noticeable with an increase in the electric potential applied. Therefore, in practice, the image forming apparatus having the above-mentioned structure cannot be applied to high speed devices, for example, a business copying machine.
In addition, similarly to the above, when the charging means for charging the surface of the photosensitive body 203 is formed by a charging roller 205 made from an elastic body, for example, rubber, as shown in FIG. 17, such a phenomenon that the electric field E.sub.Q becomes greater than a necessary value with the application of high electric potential to the transfer roller 201 occurs between the charging roller 205 and the photosensitive body 203. This is due to the same reason as mentioned above that the resistance differs between the inside of the charging roller 205 and the outside (the region of atmosphere between the transfer roller 205 and the photosensitive body 203).
Hence, in FIG. 17, when a high electric potential is applied to the charging roller 205, discharge occurs in a region other than a usual discharge region. More specifically, supposing that a region U near the contact section T between the charging roller 205 and the photosensitive body 203 is the usual discharge region, discharge occurs in a region V which is more distant from the contact section T than the region U. The diameter of a discharge path decreases with an increase in the length of a discharge path between the charging roller 205 and the photosensitive body 203, the charging electric potential varies locally on the surface of the photosensitive body 203 in such a structure. As a result, a granular pattern resulting from the unevenness of the charging electric potential appears on a printed image.
Namely, if the transfer roller 201 and charging roller 205 are called a charge supply device for supplying charge to the photosensitive body 203 and if the photosensitive body 203 to which the charge is supplied is called a body to be charged, a high electric field is formed in a region between the charge supply device and the body to be charged, where the formation of electric field is not required, when a high electric potential is applied to the charge supply device from the power supply, etc. in the above-mentioned conventional structure. Consequently, satisfactory image formation cannot be carried out.